The present invention relates to an improvement in processes for removing acid gases, such as CO.sub.2 and H.sub.2 S, from hot steam containing gas mixtures.
The industrial importance of gas scrubbing processes involving the bulk removal of acid gases, particularly CO.sub.2 and H.sub.2 S, from various raw gas mixtures is steadily increasing. As the demand for synthetic fuels and synthesis gases derived from fuel sources such as natural gas, oil and coal increases, there is an ever-increasing need for efficient processes for the removal of CO.sub.2 and/or H.sub.2 S from the raw gas mixtures that are generated. For example, in the reforming of natural gas to produce hydrogen for ammonia synthesis or hydrogenation reactions, a raw gas is produced containing usually from 16% to 20 dry mole % CO.sub.2, all of which must be removed prior to the ammonia synthesis step. Likewise, in the production of synthetic methane from naphtha, fuel oil or coal, the starting material is subjected to reforming or partial oxidation, producing a raw gas containing, e.g., from 20% to as much as 50% of CO.sub.2, together with smaller amounts of H.sub.2 S where a sulfur-containing starting material is employed.
The acid gas containing mixtures produced by such processes as steam-reforming and partial oxidation are at elevated temperatures (and usually at elevated pressures) and contain large amounts of steam. Good thermal efficiency demands the efficient recovery and utilization of the heat content of such raw gases. In this connection, the extent to which such heat content can be efficiently utilized to provide the energy required to remove the large quantities of acid gases they contain is a highly significant factor in determining the overall energy efficiency of the system.
In modern practice, the most widely used process for the bulk removal of CO.sub.2 and H.sub.2 S from such gas mixtures involves scrubbing of the gas with aqueous alkaline scrubbing solution. The scrubbing solution is continuously recirculated between an absorption stage where the acid gases are absorbed, and a regeneration stage in which the acid gases are desorbed from the solution by means of steam-stripping. For most applications, efficient types of such cyclic processes utilize a substantially isothermal absorption and regeneration cycle, i.e. the absorption and regeneration stages are operated at or close to the same temperature, e.g. a temperature in the vicinity of the atmospheric boiling temperature of the scrubbing solution. By eliminating the heating and cooling that is required by non-isothermal processes, heat losses are greatly reduced.
In any such process, whether isothermal or non-isothermal, the major energy requirement in the process is the stripping steam for regenerating the solution, and it is accordingly highly desirable to reduce the regeneration heat requirements and/or derive such regeneration heat from heat sources that may have little or no utility for other purposes.
Conventional sources of at least part of the regeneration heat employed in the prior art include that present in raw feed gas. It has been particularly desirable in the past to recover as much low-level heat content of the raw process gas as possible for regeneration, so that the higher energy level heat in the process gas can be utilized for other purposes.
For example, U.S. Pat. No. 3,823,222 (referred to herein as Benson I) discloses that the hot, steam-containing feed gas is passed in series through two heat exchangers, the first of which boils water to produce medium-pressure steam to operate a steam ejector, and the second of which raises relatively low-pressure steam in a heat exchanger where scrubbing solution is heated. The steam ejector is employed to raise additional low-pressure steam by subjecting regenerated scrubbing solution to a reduced pressure, and then to compress such low-pressure steam and inject it into the regenerator as additional stripping steam. However, the process gases, in passing through the two heat exchangers to recover the waste heat available therein, are cooled sufficiently to condense most, or at least a part of the steam present in the same. This condensed steam, referred to herein as unstripped process condensate, typically contains dissolved CO.sub.2 and other impurities such as alcohols, ammonia, and/or amines (the latter two being normally present in raw ammonia plant syn gas which has a substantial nitrogen content). In Benson I, the unstripped process condensate and impurities are not recovered before the process feed gases are fed to the absorber. Consequently, substantial amounts of water will accumulate in the cyclic system, either drastically diluting the scrubbing solution or requiring large amounts of external heat input to vaporize the excess water just to maintain the water balance in the system.
U.S. Pat. No. 4,160,810 (hereinafter referred to as Benson II) discloses in FIG. 1 the use of two heat recovery steps to make stripping steam for regeneration of the alkaline scrubbing solution.
In accordance with FIG. 1 of Benson II, the first heat recovery step involves an indirect heat exchange between the hot feed gas and the scrubbing solution, thereby heating the scrubbing solution to its boiling point and producing steam that is utilized in the regeneration stage as stripping steam. The partially cooled hot gas is then conducted to a second heat exchanger where the gas is passed in heat-exchange relationship with water (which is preferably water condensate produced in the scrubbing process). This second heat-exchange step may be carried out by direct or indirect contact of the gas with the water. In the course of this second heat-exchange step, the water is subjected a reduced pressure, thus lowering its boiling temperature, and the water of reduced boiling temperature is at the same time brought into heat-exchange relationship with the hot feed gas. The steam produced by heating of the water under reduced pressure is generated at a lower pressure than the pressure in the regeneration stage, and this low-pressure steam is compressed to a level at least equal to that in the regeneration stage and injected into the regenerator as stripping steam. However, the unstripped hot process condensate is removed from knockout pot 43A of FIG. 1 in one embodiment (the subsequent use thereof being unspecified), or in another embodiment, it may be introduced into a separate flash tank with a suitable pressure letdown valve between the knockout pot and the flash tank. The flash tank receiving the unstripped process condensate is connected to the suction side of a compressor causing low pressure steam to flash off from the condensate. This steam is then compressed and fed into the bottom of the regenerator (see col. 17 lines 4 et seq). Thus, in the first embodiment the unstripped process condensate containing impurities presumably could be discharged into the environment potentially creating ecological problems. In the second embodiment, while the low level heat content of the unstripped condensate has been recovered, most of the impurities still remain therein since mere flashing is not sufficient to purify the process condensate. Thus, in Benson II one is still faced with the problem of what to do with the substantially unstripped process condensate. Discharging it to the environment creates ecological problems. The use of unstripped process condensate as boiler feed water is limited by the corrosion problems caused by the impurities present therein at high temperatures and pressures. Thus, the use of unstripped process condensate as boiler feed water in high pressure, high temperature boilers is disadvantageous, and from an economic standpoint precluded.
In view of the above, Benson II provides no suggestion of how to approach the problem of disposal of the unstripped process condensate.
In conventional practice, the process condensate is stripped by direct or indirect (using a reboiler) heat exchange with steam or some other fluid. Typically the overhead vapors are vented or condensed against cooling water. This represents a significant energy consumption since the heat in the overhead stripper vapors are wasted.
To help reduce wasted energy consumption, alternative schemes have been developed. Thus, it is known to feed the process condensate stripper overhead vapors to the regeneration stage to permit the heat contained therein to assist in regenerating the scrubbing solution used to remove CO.sub.2, H.sub.2 S and the like from the process feed gas. These vapors are added to the regeneration stage as supplemental stripping steam. This conserves some of the energy in the process condensate stripper overhead vapors. However, in order to enter the scrubbing solution regenerator, the process condensate stripper overhead must be at a pressure level above atmospheric (particularly if the scrubbing solution regenerator overhead CO.sub.2 is to be processed for sale or to make urea). The elevated pressure necessary in the process condensate stripper in turn significantly raises the process condensate stripper bottoms temperature. A higher bottoms temperature in the stripper requires an associated higher heating fluid temperature for stripping the process condensate, since the temperature differential between the heating fluid and that of the stripper bottoms must be great enough to provide the driving force for the heat exchange. This requirement prevents the use of very low level waste heat (which has little or no value for other applications) for stripping the process condensate.
U.S. Pat. No. 4,198,378 discloses several embodiments for the purification of boiler feed waters. In some of these embodiments, the boiler feed waters include the process condensate. However, in none of these embodiments is low level heat used to strip process condensate under a pressure below that present in the regeneration stage, with the condensate stripper overhead vapors being fed to the regeneration stage. For example, in FIG. 2 process condensate can be fed to Column H from separator S for partial purification with air, or other gases, and then passed to Column D via line 26 where it is stripped (degassed) with steam. In all instances, Column D is maintained at a pressure at least equal to that of the regeneration Column B (e.g. col. 4 lines 60 et seq) or at a pressure higher than that of regeneration Column B (e.g. col. 6 lines 60 et seq). Heat supplied to Column D can be derived from high or medium temperature steam or by hot process gas preferably delivered at the maximum temperature corresponding to the outlet temperature from the CO conversion apparatus. Each of these heat sources for Column D reflects the high temperature which must be possessed thereby to obtain the necessary thermal driving force for efficient heat transfer due to the pressures and temperatures present in Column D. Likewise, in accordance with FIG. 3 of the aforenoted patent, it is known that one can modify the scheme of FIG. 3 by feeding the process condensate (which is sewered in FIG. 3) to Column D where it is stripped and flashed to provide part of the motive steam used in a steam ejector which is fed to the regeneration column as stripping steam. However, the still higher pressures necessary to generate motive steam for the ejector would increase even further the temperature requirements of the heat source to Column D (col. 7 lines 3 et seq.)
For further background on CO.sub.2 removal systems, see U.S. Pat. Nos. 3,101,996; 3,288,557; 3,714,327; 3,962,404; 4,073,863; and the two papers presented at the American Institute of Chemical Engineers, 72nd Annual Meeting, Nov. 25-29, (1979) by Crabs et al entitled "Energy Savings for Carbon Dioxide Removal Systems", and by Stokes, J., entitled "The Economics of CO.sub.2 Removal in Ammonia Plants"; none of which disclose the present invention.
In contrast, the present invention maintains the pressure of the process condensate stripper at a relatively low level thereby reducing the temperatures in the stripper bottoms. This reduction in the stripper bottoms temperature permits the use of a very low level heat source for stripping. The stripper overhead vapors are then compressed and fed at the resulting elevated pressure to the regeneration stage where the compressed vapors serve as supplemental stripping steam for regeneration of the scrubbing solution. The present invention therefore renders it possible to recover additional low level heat (which would otherwise typically be wasted), use it to purify the process condensate, and at the same time, use the resulting process condensate stripper overhead steam for regeneration. While the means for compressing the steam uses some energy, it is much less than the energy it saves. The purification of the process condensate permits it to be economically employed as boiler feed water in high temperature, high pressure boilers, or to be discharged to the environment with less adverse ecological impact than unstripped process condensate.